The present invention generally relates to materials and processes for producing directionally-solidified castings, and particularly to a process and apparatus capable of reducing defects in alloys cast as long single-crystal (SX) and directionally-solidified (DS) articles, including but not limited to components of gas turbines and other high temperature applications.
Components of gas turbines, such as blades (buckets), vanes (nozzles) and combustor components, are typically formed of nickel, cobalt or iron-base superalloys characterized by desirable mechanical properties at turbine operating temperatures. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is an ongoing effort to develop components, and particularly turbine buckets, nozzles, and combustor components, that are capable of withstanding higher temperatures. As the material requirements for gas turbine components have increased, various processing methods and alloying constituents have been used to enhance the mechanical, physical and environmental properties of components formed from superalloys. For example, buckets, nozzles and other components employed in demanding applications are often cast by unidirectional casting techniques to have directionally-solidified (DS) or single-crystal (SX) microstructures, characterized by an optimized crystal orientation along the crystal growth direction to produce columnar polycrystalline or single-crystal articles.
As known in the art, directional casting techniques for producing SX and DS castings generally entail pouring a melt of the desired alloy into an investment mold held at a temperature above the liquidus temperature of the alloy. One such process is represented in FIGS. 1 and 2 as an apparatus 10 that employs a Bridgman-type furnace to create a heating zone 26 surrounding a shell mold 12, and a cooling zone 42 beneath the mold 12. The zones 26 and 42 may be referred to as “hot” and “cold” zones, respectively, which as used herein denotes their temperatures relative to the melting temperature of the alloy being solidified. The mold 12 has an internal cavity 14 corresponding to the desired shape of a casting 32 (FIG. 2), represented as a turbine bucket. As such, FIG. 1 represents the cavity 14 as having regions 14a, 14b and 14c that are configured to form, respectively, an airfoil portion 34, shank 36, and dovetail 38 (FIG. 2) of the casting 32. The cavity 14 may also contain cores (not shown) for the purpose of forming cooling passages within the casting 32. The mold 12 is shown secured to a chill plate 24 and placed in the heating zone 26 (Bridgman furnace). The heating zone 26 heats the mold 12 to a temperature above the liquidus temperature of the alloy. The cooling zone 42 is directly beneath the heating zone 26, and operates to cool the mold 12 and the molten alloy 16 within by conduction, convection and/or radiation techniques. For example, the cooling zone 42 may be a tank containing a liquid cooling bath 46, such as a molten metal, or a radiation cooling tank that may be evacuated or contain a gas at ambient or cooled temperature. The cooling zone 42 may also employ gas impingement cooling or a fluidized bed.
An insulation zone 44 defined by a baffle, heat shield or other suitable means is between and separates the heating and cooling zones 26 and 42. The insulation zone 44 serves as a barrier to thermal radiation emitted by the heating zone 26, thereby promoting a steep axial thermal gradient between the mold 12 and the cooling bath 46. The insulation zone 44 has a variable-sized opening 48 that, as represented in FIG. 1, enables the insulation zone 44 to fit closely around the shape of the mold 12 as it is withdrawn from the heating zone 26, through the insulation zone 44, and into the liquid cooling bath 46.
Casting processes of the type represented in FIGS. 1 and 2 are typically carried out in a vacuum or an inert atmosphere. After the mold 12 is preheated to a temperature above the liquidus temperature of the alloy being cast, molten alloy 16 is poured into the mold 12 and the unidirectional solidification process is initiated by withdrawing the base of the mold 12 and chill plate 24 downwardly at a fixed withdrawal rate into the cooling zone 42, until the mold 12 is entirely within the cooling zone 42 as represented in FIG. 2. The insulation zone 44 is required to maintain the high thermal gradient at the solidification front to prevent nucleation of new grains during the directional solidification processes. The temperature of the chill plate 24 is preferably maintained at or near the temperature of the cooling zone 42, such that dendritic growth begins at the lower end of the mold 12 and the solidification front travels upward through the mold 12. The casting 32 grows epitaxially from a small block 28 at the bottom of the mold 12. The block 28 may be, for example, a cylindrical chill block or a conical seed piece from which a single crystal forms from a crystal selector 30, for example, a pigtail sorting structure. The columnar single crystal becomes larger in the enlarged section of the cavity 14. A bridge 40 connects protruding sections of the casting 32 with lower sections of the casting 32 so that a unidirectional columnar single crystal forms substantially throughout the casting 32. The casting 32 is typically deemed to be a substantially columnar single crystal if it does not have high angle grain boundaries, for example, greater than about twenty degrees.
Mechanical properties of DS and SX articles depend in part on the avoidance of high-angle grain boundaries, equiaxed grains, and other potential defects that may occur as a result of the directional solidification process. As an example, small dendrite arm spacing is usually desired to avoid casting defects such as stray grains, slivers and freckles, and to improve the uniformity of strengthening phases and improve mechanical properties at service temperatures of the article. A small dendrite spacing can be effectively obtained by a steep thermal gradient at the growth interface during directional solidification. In a conventional Bridgman apparatus, the temperature of the heating zone 26 is generally maintained at a temperature of about 300 to about 400° F. (about 160 to about 220° C.) above the liquidus temperature of the alloy in order to obtain a sufficiently high thermal gradient. However, detrimental effects can inevitably occur if the shell mold 12 is held at an excessively high temperature within the heating zone 26 for an extended period of time. Such dimensional defects may result from creep movement and deformation of the mold 12 and any cores used in the casting process, and surface finish defects resulting from interactions between the molten alloy 16 and the mold 12 and cores. Such interactions are particularly possible if the alloy contains elements that are reactive at high temperatures (“reactive elements”), such as yttrium, zirconium and hathium, and to a lesser extent other elements such as tantalum, tungsten, rhenium, and titanium, which are also often referred to as being reactive. Because superalloys typically contain reactive elements, a common practice is to protect the surface of the mold 12, which is typically formed of a refractory material such as alumina or silica, with a facecoat, a nonlimiting example of which contains yttria (Y2O3). While effective in reducing reactions with many alloy compositions, protective facecoats do not address other potential defects that may occur during the solidification process, including dimensional defects resulting from extended stays at excessive temperatures.